Heretofore, a spark plug which ignites a mixture using a spark discharge has been used in a combustion device such as an internal combustion engine. Also, in recent years, in order to comply with a demand for higher power and lower fuel consumption in the combustion device, a plasma jet spark plug has been proposed as a spark plug with which it is also possible to more reliably ignite a lean mixture with a fast-spreading combustion and a higher ignition limit air/fuel ratio.
In general, a plasma jet spark plug includes a hollow cylindrical insulating body having an axial hole, a center electrode inserted into the axial hole in a condition in which the leading end face of the center electrode is withdrawn below the leading end face of the insulating body, a metal shell disposed on the outer periphery of the insulating body, and an annular ground electrode joined to a leading end portion of the metal shell. Also, the plasma jet spark plug has a space (a cavity portion) formed by the leading end face of the center electrode and the inner peripheral surface of the axial hole, wherein the cavity portion is caused to communicate with the exterior via a through hole formed in the ground electrode. For example, refer to JP-A-2010-218768 (Patent Document 1).
With this kind of plasma jet spark plug, an ignition of a mixture is carried out in the following way. Firstly, a voltage is applied to a gap formed between the center electrode and ground electrode, thereby causing a spark discharge in the gap. After that, a gas in the cavity portion is plasmanized by supplying power to the gap, generating plasma inside the cavity portion. Then, the generated plasma is emitted from an opening of the cavity portion, thereby carrying out an ignition of a mixture.